This invention relates to a valve for controlling the flow rate of a fluid in response to a digital electrical signal. Such valves are used for blending and mixing liquids, e.g., in the dispensing of gasoline.
A digitally controllable valve has a main diaphragm valve connected to two pilot solenoid valves. The pilot valves are independently opened and closed to control the volume in the chamber immediately over the main diaphragm or piston.
To obtain constant volume rate of primary flow, i.e., flow through the seat of the main valve, both pilot valves are closed via their respective solenoids and the outlet pilot valve connected to the outlet port of the main valve is pulsed open to gradually relieve the fluid pressure from behind the diaphragm. In response to the pulsing, the diaphragm moves up by a controlled distance depending upon the width of the pulse and the number of pulses applied, and fluid is allowed to escape from behind the main diaphragm. By this method, the diaphragm can be incrementally raised off of the main valve seat to allow controlled opening in digital stages. To close the main valve, the outlet pilot valve is closed and the inlet pilot valve is opened by pulsing its solenoid. This now restores fluid above the diaphragm to digitally lower the diaphragm in controlled increments.
In such digitally controllable valves, a main valve member mounted on a diaphragm is movable within an opening in a main valve seat communicating with a main valve outlet port to define a variable cross sectional passageway through which fluid can flow from the main valve inlet port to its outlet port. The cross sectional area of the passageway is controlled by raising and lowering the main valve member from a sealing position in engagement with the main valve seat.
When the main valve member is in engagement with the main valve seat, fluid flow from the inlet port to the outlet port through the main valve seat is prevented. As the main valve member is lifted off of the main valve seat, fluid from the inlet port flows through the passageway between the main valve member and seat. The greater the displacement of the main valve member from the main valve seat, the larger is the passageway, and hence the rate of flow.
It is also known in the prior art to connect each of the inlet and outlet ports of the main valve to a reservoir on a side of the diaphragm opposite the main valve seat, through respective inlet and outlet solenoid pilot valves to control the fluid pressure above the diaphragm. The pilot valves are controlled by electrical power supplies which produce voltage pulses of width and frequency selected to incrementally raise and lower the main valve member to achieve a desired flow rate versus time profile. In certain applications, e.g., in blending and mixing valves used in the gasoline dispensing industry, it is desirable to be able to increase and decrease flow rates symmetrically, that is, to have the flow rate of a one constituent of a mixture through one valve increase at the same rate that the flow rate of another constituent fluid passed through another valve decreases so that the net flow rate of the blended mixture remains constant.
Referring to FIG. 1 there is shown a prior art, digitally controllable valve 1 having a main valve 3, an inlet solenoid pilot valve 5, and an outlet pilot solenoid valve 7. Pressure above a diaphragm 9 is controlled through two bleed passageways 11, 13 respectively connected to an outlet port of the inlet pilot valve 5 and an inlet port of the outlet pilot valve 7. Fluid enters the region 8 above the diaphragm 9 from the inlet pilot valve 5 and exits that region 8 through the outlet pilot valve 7. During a "low flow" mode of operation, both the inlet and outlet valves 5, 7 are opened and the main valve 3 closes to shunt all fluid flow through the pilot valves 5, 7.
Closure of the main valve 3 in response to the opening of both pilot valves 5, 7 is achieved by making the minimum cross sectional area of fluid flow through the inlet valve greater than the minimum cross sectional area of fluid flow through the outlet valve. This can be accomplished by making the diameter of the the inlet valve seat opening greater than the diameter of the outlet valve seat opening.
The result is that fluid from the inlet pilot valve 5 enters the region 8 above the diaphragm 9 faster than it can exit causing an increase in pressure above the diaphragm 9 which urges the diaphragm 9 to engage and seal a main valve seat 15 thereby preventing fluid flow within the main valve 3.
The introduction of this incongruence between the inlet and outlet pilot valves 5, 7 prevents symmetrical operation of the digitally controllable valve 1 for increasing and decreasing flow rate. That is, if two identical digitally controllable valves used to blend two constituent fluids of a mixture are energized by the same signal, one in an increasing flow rate mode and the other in a decreasing flow rate mode, the magnitudes of the rates of increasing and decreasing flow of the respective constituents will be unequal and the net flow rate of the blend will vary.
In one of the two identical digitally controllable valves 1 which is dispensing the constituent fluid with an increasing flow rate, its inlet valve is closed to prevent entry of fluid into the region 8 above the diaphragm 9 and the outlet valve is opened to enable the fluid above the diaphragm 9 to exit through the outlet port thereby causing the main valve member to move away from the main valve seat to increase the rate of flow. A desired flow rate vs. time profile is achieved by energizing the solenoid coil of the outlet pilot valve 7 with a train of voltage pulses having a corresponding pulse width and frequency.
In the other of the two identical digitally controllable valves 1 which is dispensing its constituent fluid with a decreasing flow rate, its inlet valve is opened to permit entry of fluid into the region 8 above the diaphragm 9 and its outlet valve is closed to prevent the fluid above the diaphragm 9 from exiting through the outlet port thereby causing the valve member to move toward from the valve seat to decrease the rate of flow. It is desired to achieve a complementary profile with respect to the increasing flow rate of the other constituent, that is, to have the flow rate decrease in accordance with a profile identical to but 180 degrees out of phase with the flow rate vs. time profile of the increasing flow rate constituent fluid. Due to the incongruence in the flow path dimensions of the valves 5, 7, this cannot be achieved by pulsing the outlet pilot valve 7 of one digitally controllable valve 1 for increasing the flow rate of its constituent with the same signal used to pulse the inlet pilot valve 5 of the other digitally controllable valve 1 for decreasing the flow crate of the its constituent fluid.
Hence in prior art digitally controllable valves, the achievement of complementary increasing and decreasing flow rate profiles has required that the electrical signals used to increase flow rate be altered with respect to those used to decrease flow rate to compensate for the incongruence between the bleed passageways leading from above the diaphragm 9 to the inlet and outlet pilot valves 5, 7. Such compensation requires duplication of hardware and/or use of complex circuitry and is difficult to achieve.